1. Field of the Invention
The present invention relates to methods for selecting piezoelectric transformer characteristics, and more particularly, the present invention relates to a method for selecting a characteristic of a piezoelectric transformer which is used, for example, as a driving power source of a backlight inverter of a liquid crystal display, a camera flash device, or suitable devices.
2. Description of the Related Art
A Rosen-type piezoelectric transformer is one of the generally known piezoelectric transformers. This piezoelectric transformer includes a piezoelectric body unit which is formed by laminating ceramic sheets made of a piezoelectric ceramic material such as lead zirconium titanate (PZT) and then baking the laminated sheets. The piezoelectric body unit includes a driving unit (primary side) and a generating unit (secondary side).
There are three factors that determine the efficiency and the step-up ratio of the piezoelectric transformer having the above construction: the polarizability of the primary side, the polarizability of the secondary side, and the internal electrode film thickness of the primary side. Here, the polarizability is a proportion of the amount of polarization in which the amount of polarization in a saturation state is 1 (=100%). When these polarizabilities and the internal electrode film thickness are desired to be examined, the primary side and the secondary side of the piezoelectric transformer must be separately cut into test pieces to determine the electromechanical coupling factors thereof. However, since the piezoelectric transformer cannot be destroyed during a shipping inspection in a mass-production process, this method cannot be used. Accordingly, conventionally, defective or non-defective piezoelectric transformers are selected based on evaluations of an efficiency-versus-frequency characteristic and a step-up-ratio-versus-frequency characteristic.
However, when the efficiency-versus-frequency characteristic or the step-up-ratio-versus-frequency characteristic is evaluated to select the piezoelectric transformer, the characteristic must be evaluated in a state in which the primary side and the secondary side of the piezoelectric transformer are electrically connected. Particularly, a connection which is secure and which also allows the great number of possible vibrations is required for the secondary side which exhibits a high level of vibration. Therefore, it is difficult to evaluate the piezoelectric transformer in an isolated state before it is mounted on a substrate, in a casing, or other such substrate. In most cases, when the piezoelectric transformer is evaluated, it is mounted on the substrate or in an inverter circuit. In addition, since use of this method is time-consuming, it is unproductive to perform selection of the characteristic using this method during the mass-production process.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for selecting a piezoelectric transformer characteristic in which the piezoelectric transformer can be evaluated in an isolated state and the selecting time can be reduced.
According to a first preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof in an open state, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant frequency fr of an input-impedance-versus-frequency characteristic of the piezoelectric transformer, and selecting the characteristic of the piezoelectric transformer based on the value of the measured resonant frequency fr.
According to a second preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof in an open state, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a bandwidth of an input-impedance-versus-frequency characteristic of the piezoelectric transformer in which the bandwidth is determined by subtracting a resonant-frequency fr from an antiresonant-frequency fa, and selecting the characteristic of the piezoelectric transformer based on the value of the measured bandwidth.
According to a third preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof in an open state, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant resistance Zr from an input-impedance-versus-frequency characteristic of the piezoelectric transformer, and selecting the characteristic of the piezoelectric transformer based on the value of the measured resonant resistance Zr.
According to a fourth preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof in an open state, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant frequency fr, a bandwidth, and a resonant resistance Zr from an input-impedance-versus-frequency characteristic of the piezoelectric transformer in which the bandwidth is determined by subtracting a resonant-frequency fr from an antiresonant-frequency fa, and selecting the characteristic of the piezoelectric transformer based on the values of the measured resonant frequency fr, the bandwidth, and the resonant resistance Zr.
According to a fifth preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof short-circuited, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant frequency fr of an input-impedance-versus-frequency characteristic of the piezoelectric transformer, and selecting the characteristic of the piezoelectric transformer based on the value of the measured resonant frequency fr.
According to a sixth preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof short-circuited, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a bandwidth of an input-impedance-versus-frequency characteristic of the piezoelectric transformer in which the bandwidth is determined by subtracting a resonant-frequency fr from an antiresonant-frequency fa, and selecting the characteristic of the piezoelectric transformer based on the value of the measured bandwidth.
According to a seventh preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof short-circuited, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant resistance Zr of an input-impedance-versus-frequency-characteristic of the piezoelectric transformer, and the characteristic of the piezoelectric transformer based on the value of the measured resonant resistance Zr.
According to an eighth preferred embodiment of the present invention, a method for selecting a piezoelectric transformer characteristic includes the steps of connecting a primary-side driving section of a piezoelectric transformer to a high-frequency generator while leaving a secondary-side generating section thereof short-circuited, causing the high-frequency generator to sequentially generate and sweep a high-frequency signal over a predetermined frequency range, measuring a resonant frequency fr, a bandwidth, and a resonant resistance Zr of an input-impedance-versus-frequency characteristic of the piezoelectric transformer in which the bandwidth is determined by subtracting a resonant-frequency fr from an antiresonant-frequency fa, and selecting the characteristic of the piezoelectric transformer based on the values of the measured resonant frequency fr, the bandwidth, and the resonant resistance Zr.
The polarizability condition of the secondary-side generating section of the piezoelectric transformer can be examined by measuring the resonant frequency from the input-impedance-versus-frequency characteristic. The polarizability condition of the primary-side driving section of the piezoelectric transformer can be examined by measuring the bandwidth. The film thickness condition of internal electrodes of the primary-side driving section can be examined by measuring the resonant resistance. Therefore, instead of measuring the efficiency-versus-frequency characteristic or the step-up-ratio-versus-frequency-characteristic, by measuring the resonant frequency, the bandwidth, or the resonant resistance from the input-impedance-versus-frequency characteristic, the piezoelectric transformer having a predetermined characteristic can be selected. Since only the input-impedance characteristic of the piezoelectric transformer is measured, the measuring time is greatly reduced.
When the input-impedance-versus-frequency characteristic is measured in a case in which the secondary-side generating section of the piezoelectric transformer is in an open state, the piezoelectric transformer can be measured in an isolated state (that is, the transformer does not have to be mounted to a substrate or an inverter circuit). Therefore, even though the piezoelectric transformer that does not meet the selection criteria and standards is produced, only the piezoelectric transformer is wasted as a defective product, which is contrary to a conventional case in which the surface-mounting substrate or the casing provided with the defective piezoelectric transformer is also wasted. Therefore, significant cost savings are achieved with preferred embodiments of the present invention.
Other features, elements, steps, processes and advantages of the present invention will become apparent from the detailed description of preferred embodiments with reference to the attached drawings.